AutoValue Extensions

AutoValue is a tool that lets you remove a lot of the boilerplate associated with writing value types in Java. It does this by using an annotation processor to generate a well-behaved, immutable implementation of an abstract class.

So, given the abstract class Foo, AutoValue will then generate the implementation of the class called AutoValue_Foo.

@AutoValue abstract class Foo {
  public abstract String bar();

  public static Foo create() {
    return new AutoValue_Foo();
  } 
}

final class AutoValue_Foo extends Foo {  
  AutoValue_Foo() {...}
  public String bar() {...}
  @Override public String toString() {...}
  @Override public boolean equals(Object o) {...}
  @Override public int hashcode() {...}
}

So there are some immediately obvious advantages to using AutoValue;

  • No runtime cost (since it's all compile time code generation).
  • The generated class is immutable.
  • No boilerplate equals()/hashcode()/toString() method implementations.

However, the AutoValue alone is insufficient for Android development because:

  • AutoValue_Foo can't be Parcelable.
  • AutoValue_Foo can't be deserialized automatically by Moshi/Gson (because the class is immutable).

Enter AutoValue Extensions

AutoValue 1.2 added an Extensibility API that allows you to add custom behavior, like Parcelable, to an AutoValue class.

Parcelable AutoValue Extension

First we define our AutoValue class Foo. Note that our class Foo implements Parcelable. This tells the Parcelable Extension that it is applicable to this class and it should participate in code generation.

@AutoValue abstract class Foo implements Parcelable {
  public abstract String bar();
}

The AutoValue annotation processor will generate a subclass called $AutoValue_Foo, which contains all of the normal AutoValue code. Note that it is designated as abstract because the Parcelable Extension still needs to fulfill the Parcelable interface.

abstract class $AutoValue_Foo extends Foo {  
  $AutoValue_Foo() {...}
  public String bar() {...}
  @Override public String toString() {...}
  @Override public boolean equals(Object o) {...}
  @Override public int hashcode() {...}
}

The ParcelableExtension will then generate AutoValue_Foo that extends $AutoValue_Foo. This lets the Parcelable Extension implement the Parcelable interface properly.

final class AutoValue_Foo extends $AutoValue_Foo {  
  @Override public int describeContents() {...}
  @Override public void writeToParcel(Parcel out, int flags) {...}
  public static final Parcelable.Creator<AutoValue_Foo> CREATOR = ...
}

So the full class hierarchy looks like:

    Foo
     ^
$AutoValue_Foo
     ^
 AutoValue_Foo

Also note that the final generated class is always called AutoValue_Foo, which means that nothing in the consuming code needs to change!

TL;DR

AutoValue 1.3 has extensions, they are a powerful way to reduce the boilerplate code associated with value types. Use them!


See